1. Field of the Invention
The present invention relates to a display device, and more particularly, to an in-plane switching mode liquid crystal display (LCD) device and a method for manufacturing the same, in which a liquid crystal molecule is controlled by an electric field parallel to a substrate.
2. Discussion of the Related Art
An ultra thin sized flat panel display having a display screen with a thickness of several centimeters, especially LCD devices, are widely used in monitors for notebook computers, spacecraft, and aircraft.
Such an LCD device has low power consumption and is easy to transport. In this respect, LCD devices are receiving much attention as an advanced display device that can substitute for a cathode ray tube (CRT).
The LCD device includes a thin film transistor (TFT) substrate, a color filter substrate, and a liquid crystal layer sealed between the TFT substrate and the color filter substrate. The LCD is a non-light-emitting device that can obtain image effect based on electric optical characteristics of the liquid crystal layer.
In other words, a TFT array and a pixel electrode are formed on the TFT substrate, while a black matrix pattern, a color filter layer, and a common electrode are formed on the color filter substrate. The TFT substrate and the color filter substrate are bonded to each other by a sealant such as epoxy resin.
A driving circuit is connected with the TFT substrate using a tape carrier package (TCP) as a medium. The driving circuit generates various control signals and signal voltages to display images.
Development and application of TFT-LCDs have accelerated in accordance with an increase in the dimensions and increase of the resolution. To increase the productivity and ensure the low cost, many efforts have been continued in view of simplified process steps and improvement of yield. However, in spite of the trend towards LCDs with large area, a problem arises in that contrast ratio varies depending on viewing angle.
To solve this problem, various LCDs, such as a twist nematic LCD provided with a retardation film and a multi-domain LCD, have been proposed. However, a problem still arises in that contrast ratio and color are varied depending on a viewing angle.
To ensure a wide-viewing angle, an in-plane switching mode LCD device that controls alignment of a liquid crystal by an electric field parallel to a substrate has been proposed.
FIG. 1 is a plan view illustrating a related art in-plane switching mode LCD device, and FIG. 2 is a sectional view taken along line I–I′ of FIG. 1.
As shown in FIG. 1, gate lines 11 are arranged to cross data lines 13 so that a plurality of pixel regions are defined. A common electrode 15 and a data electrode 17 are formed within the pixel region in parallel. For reference, FIG. 1 shows a unit pixel.
The common electrode 15 is of metal, while the data electrode 17 is of metal, or indium tin oxide (ITO), which is a transparent conductive material.
The common electrode 15 and the gate line 11 are formed by the same process while the data line 13 and the data electrode 17 are formed by the same process.
In the related art in-plane switching mode LCD device, as shown in FIG. 2, the common electrodes 15 are formed on a first substrate 10, and a gate insulating film 16 is formed on the first substrate 10 including the common electrodes 15. The data electrode 17 is formed on the gate insulating film 16, and a passivation film 18 is formed on an entire surface including the data electrode 17.
Meanwhile, a light-shielding layer 21 is formed on a second substrate 10a, which opposes the first substrate 10, so as to prevent light from being leaked out to the data line 13, the gate line 11 and a TFT formed on the first substrate 10. A color filter layer 23 is also formed on the second substrate 10a to display color. A liquid crystal layer 25 is formed between the first substrate 10 and the second substrate 10a. 
In the related art in-plane switching mode LCD device, as shown in FIG. 2, an in-plane electric field parallel to the substrates is generated between the data electrode 17 and the common electrode 15. The in-plane electric field is determined considering a dielectric constant 1 of the gate insulating film 16, a dielectric constant 2 of the passivation film 18, and a dielectric constant 3 of the liquid crystal layer 25. At this time, a distance L1 is maintained between the common electrode 15 and the data electrode 17.
In the aforementioned related art in-plane switching mode LCD device, if a driving voltage is applied from an external driving circuit (not shown), the in-plane electric field parallel to the substrates is generated between the data electrode 17 and the common electrode 15. A liquid crystal molecule aligned within the liquid crystal layer 25 by the in-plane electric field is rotated along the in-plane electric field in parallel with the substrates. As a result, an amount of light that passes through the liquid crystal layer 25 is controlled and gray is displayed in a state when the liquid crystal molecule is parallel to the substrate. For this reason, the difference of light transmittivity depending on viewing angle is reduced.
However, the related art in-plane switching mode LCD device has several problems.
Since the distance between the common electrode and the data electrode is large, a threshold voltage Vth increases. For this reason, response time and a dynamic range of a drive integrated circuit (IC) increase. In other words, since the distance between the common electrode and the data electrode is large, a cyano (CN) based liquid crystal having high viscosity and dielectric constant is required to improve response time. Since the CN based liquid crystal has high polarity, it is not better than a fluorine (F) based liquid crystal in view of contamination and reliability. To improve reliability, the F based liquid crystal may be used. However, in the related art structure, it is not desirable that the F based liquid crystal is used. This is because the F based liquid crystal can improve reliability, but is less effective than the CN based liquid crystal in view of the dynamic range of the drive IC and response time.